one thing i have noticed is that a jailbroken iphone 3G is very very laggy, very very slow and very annoying. i jailbroke my iphone 3GS (for a day mind you then i restored lol) and it was fast as hell, not a single issue.

games run ridiculously smooth on the 3GS and the 3GS jailbroken can play game boy games at 120fps and psx games at around 30fps (which is normal speed for a psx game)